Infrared imaging apparatus

ABSTRACT

An infrared scanning system comprising input optics including objective and field lenses, output optics including a relay lens and a linear array of detectors, a scanner including a hollow inwardly reflecting polygonal prism rotated about its longitudinal axis, which is aligned with the axis of the input optics, and further optical components including a relay lens, a strip mirror, and first and second folding mirrors, which are contained within the outline of the prism, and with respect to which the prism rotates.

[ Apr. 17,1973

United States Patent Abel INFRARED IMAGING APPARATUS Inventor: lrving R.Abel, Lexington, Mass. Pmrary Exam" 1er|ame s L'fmfrence AssistantExammerDavis L. Willis Assignee: Honeywell Inc., Minneapolis, Minn. Anrney Charles J. Ungemach, Albin M d d d A r. 28, 1971 George W. FieldAppl. No.: 138,085

[22 Filed:

[57] ABSTRACT An infrared scanning system comprising input opticsincluding objective and field lenses, output optics including a relaylens and a linear array of detectors, a scanner including a hollowinwardly reflecting polygonal prism rotated about its longitudinal axis,which is aligned with the axis of the input optics, and further opticalcomponents including a relay lens, a strip mirror, and first and secondfolding mirrors, which are contained within the outline of the prism,and with respect to which the prism rotates.

4b.. HX HRH 3H 3 M3 3M3 3 0008 8 5/ 0G 2 0 5 "H 5 m S m A m "nu h P Tm CS m nu E "u" m T "um n A cm "H. 8 a n r T e "U" k S BN mm e D "um R E 10mmw T 77 "a W 1 m U WM ew rw 1 Uhr! M5 l1] 400 2 8 6 00 5 J5 5 6,5 .ll.33

16 Claims, 4 Drawing Figures H 3 N O 5 2 3,153723 10/1964Weiss............, 3,353,022 11/1967 Schwartz.................,....

SIGNAL PROCESSING O 000 OO PATENTEDAPR 1 71973 SHEET 2 OF 2 625m mm A mINVENTOR- IRVING R. ABE

ATTORNEY INFRARED IMAGING APPARATUS BACKGROUND OF THE INVENTION Thisinvention relates to the field of optical instruments, and moreparticularly to infrared sensitive imaging apparatus for use in a movingaircraft to perform continuous repeated transverse scans of the terrainbelow.

The use of infrared wavelengths in aerial reconnaissance is of knownadvantage, but requires some means for making the radiation perceptibleto an observer. Detectors for converting infrared energy to electricalenergy are known, and in theory if the radiation from a scene wereprojected on a mosaic of such detectors a set of electrical signalsreproducible as a continuous picture of the scene would result. Inpractice the art has not yet developed to the point where faultlessdetector mosaics are reliably producible. Reproduction of the electricalsignals also presents difficulties: if the mosaic is to be multiplexed,detector usage is extremely inefficient unless electronic signal storageis provided, with concommitent circuit complexity, while if parallelread-out and display of the mosaic is contemplated, the complexityinvolved therein is comparable to that of a multiplex system.

The alternatives to a detector mosaic system are systems in which thefield of view time-shares a single detector or a simple linear array ofdetectors, by the use of an opto-mechanical scanning arrangement.Time-sharing of a single detector by use of some form of area scanner ismost efficient as far as detector utilization is concerned, but does notprovide the sensitivity and resolution of a multiple detector systemwith a simple transverse scan. The latter system thus appears to offerthe optimum compromise between complexity and sensitivity.

The present invention has for its object to provide an improved infraredimaging system using a linear array of detectors across which the fieldof view of the instrument is cyclically swept by a novel optical scannerwhich gives a linear scan at a rapid rate with good resolution, goodstability, and good transmittence of the radiant energy, and which iscompact, easily constructed, and readily aligned.

Various other objects, advantages, and features of novelty whichcharacterize my invention are pointed out with particularity in theclaims annexed hereto and forming a part hereof. However, for a betterunderstanding of the invention, its advantages, and objects attained byits use, reference should be had to the drawing, which forms a furtherpart hereof, and to the accompanying descriptive matter, in which thereis illustrated and described a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings,

FIG. 1 is a block diagram of an infrared imaging system including myinvention;

FIG. 2 is a schematic diagram of the optics of my scanner;

FIG. 3 schematically discloses one display arrangement suitable for usewith my scanner; and

FIG. 4 is a perspective view of one embodiment of a system including myscanner.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1 there is illustratedan infrared imaging system according to my invention. Infrared radiation11 from the scene in the field of view of the instrument is supplied tothe input optics 12 of an optical assembly 13 which further includes ascanner l4 and output optics 15. The latter includes a linear array ofdetectors which for efficient operation may be cooled by a supply ofrefrigerant at 16.

Scanner 14 functions, in combination with optics 12 and 15, to sweep animage of the field of view across the detectors, and the latter thensupply a set of varying outputs on a cable 17 to signal processingapparatus 20 which may include a plurality of suitable amplifiers,filters, and other appropriate circuitry. After processing, the signalsare supplied on a cable 21 to a display 22: coordination between theoperation of the scanner and that of the display is necessary, andsynchronizing means 23 is schematically shown for accomplishing thisfunction.

Reference should now be made to FIG. 2, in which the path of radiationthrough apparatus 13 is shown: the direction changes produced by variousmirrors have been omitted for clarity. At the discretion of the designerthe various lenses may be made of single or multiple elements, and inthis respect the figure is schematic only.

Essentially collimated infrared energy from the scene is received at anobjective lens which produces a real image at the plane of a field stop31. A field lens 32 may be associated with objective lens 30 if desired.After reflection at a first folding mirror 33 the radiation passesthrough a first relay lens 34 which produces a second real image afterreflection from a rotating prism mirror 35 presently to be described.The second image is produced at the surface of a narrow strip mirror 36:after reflection therefrom and from a second folding mirror 37 theradiation passes through a second relay lens 40 which produces a thirdimage in the plane of a line array of detectors 41 responsive toinfrared radiation. All the lenses named above must of course be ofgermanium or other material transparent to radiation in the infraredband, and the mirror surfaces must also be reflective to this radiation.The detectors 41 give outputs on a like plurality of conductors 42making up output cable 17 of FIG. 1.

FIG. 3 suggests one suitable form which can be taken by display 22.Output cable 21 of signal processor 20 (FIG. 1) comprises a plurality ofconductors 43 equal in number to those in cable 17, which are connectedto a like plurality of light emitting diodes 44 arranged in the same wayas are detectors 4]. A lens 45 images the diodes on an observing screen46. In passing from the diodes to the lens the emitted radiation, now inthe visible band,'is reflected from a folding mirror 47 and a scannermoving synchronously with scanner 14, as suggested by connection 23.Thus, visual or television supervision of the scanner may be provided.

For a more detailed mechanical showing of one embodiment of my scanner13, reference should now be made to FIG. 4. The scanner is shown to havea single moving part in the form of a hollow inwardly reflecting prism50 the cross section of which is a regular polygon. For convenience ofillustration I have shown a prism having 12 reflecting faces 35, but 16faces might in some cases be more desirable: in general the number offaces is a matter of selection by the designer and may in particularcases be even or odd.

The prism or drum is mounted in any suitable fashion for rotation aboutan axis 51 normal to the polygons at their centers, and in theembodiment shown in FIG. 4 the faces all comprise planes parallel tothis axis. A slight departure from this parallel relation may be used toproduce interlaced scan lines if this is desired. Any constant speedmotor of suitable power rating may be used for driving the drum, towhich it may be geared, belted, or otherwise mechanically connected.-

Contained within the outline of drum 50 are folding mirrors 33 and 37,relay lens 34, and strip mirror 36. These elements are fixed in anysuitable means so that drum 50 rotates around them. Radiation from thescene being observed reaches the instrument in a direction aligned withaxis 51, and passes through objective lens 30 and field lens 32 to beimaged at field stop 31. This image is received by lens 34 afterreflection at mirror 33, to be directed diametrically of the drum, andlens 34 produces a second image at strip mirror 36, after reflection ofthe radiation from a mirror surface 35 of the drum back along itsoriginal path. Strip mirror 36 is located on axis 51 and is very narrow,to avoid as much as possible obscuration of the light passing from lens34 to the drum. It is preferably in the form of a cylindrical surfacewhose axis lies in a plane perpendicular to the axis of rotation. Thereflecting surface of mirror 36 is preferably tangent to axis 51. Forsimplicity of illustration mirror 36 is shown in FIG. 2 as a plane, butby giving this mirror a slight curvature correction may 'be accomplishedfor the field curvature introduced by other optical elements. Stripmirror 36 is oriented so that the light reflected therefrom movesoutwardly along a different radius of the drum, to be reflected byfolding mirror 37 and so to pass out of the confines of the scanningdrum to relay lens 40, which reimages it on the linear array ofdetectors 4].

The operation of this structure is to sweep the image of the sceneacross the array of detectors, in what might be termed a combing motion,as many times per revolu-- tion of the drum as the latter has internallyreflecting faces. During each sweep or scan each detector gives anoutput which continuously varies with the irradiation of the detector bythe scene. These outputs are suitably processed by amplification andfiltering if necessary, and are supplied to a linear array of lightemitting diodes 44 located along axis 51. Light from the diodes isreflected by drum mirror 35 and further folding mirror 47, and isfocused by lens 45 of a television camera 52 on the target of thecameratube for display as at 53. A relay lens similar -to lens 34 may be usedhere if desired.

Numerous objects and advantages of my invention have been set forth inthe foregoing description, together with details of the structure andfunction of the invention, and the novel features thereof are pointedout in the appended claims. The disclosure, however, is illustrativeonly, and changes may be made in detail especially in matters of shape,size, and arrangement of parts, within the principle of the invention,to the full extent indicated by the broad general meaning of the termsin which the appended claims are expressed.

I claim as my invention: 1. Means for scanning the field of view along aline of sight comprising, in combination:

a hollow scanning drum rotatable about a first axis,

including a plurality of inwardly reflecting surfaces substantiallyparallel to said axis; means causing rotation of said drum; firstoptical means including a first stationary folding mirror, fixed withrespect to said drum and positioned to direct optically reflectableradiation having the direction of said line of sight to impingeconvergingly 'on said reflecting surfaces successively as said drumrotates; linear array of radiation detectors fixed with respect to saiddrum; and second optical means, including a second sta tionary foldingmirror, fixed with respect to said drum and arranged to receiveradiation reflected from said surfaces and direct it toward said array,whereby during rotation of said drum successive images of said field ofview sweep traversely across said array, to produce varying outputs fromsaid light detectors. 2. Apparatus according to claim 1 in which saidsurfaces are plane. 1

3. Apparatus according to claim 1 in which said surfaces are symmetricalabout said axis.

4. Apparatus according to claim 1 in which said surfaces jointly definea regular polyhedral prism.

5. Apparatus according to claim 1 in which said first optical means alsoincludes lens means for causing said radiation to impinge converginglyon said surfaces.

6. Apparatus according to claim-1 in which said first and second opticalmeans are contained within the outline of said drum.

7. Apparatus according to claim 1 in which said delimited transversedimension to cause minimum ob scuration of said light.

9. Apparatus according to claim 6 in which said further reflector is aportion of the surface of a cylinder of which the axis lies in a planeperpendicular to said first axis, and which is tangent to said firstaxis.

10. Apparatus according to claim 1 including further means for causingsaid reflected radiation to image on said array. 3

1 1. Apparatus according to claim 5 in which said lens means includes arelay lens located within the outline of said drum between said firstfolding mirror and said surfaces.

12. Apparatus according to claim 8 in which said further means includesa strip mirror located within the outline of said drum and a relay lens.1

13. Apparatus according to claim 1 in which said surfaces slightlydepart in opposite directions alternately from a parallel relation tosaid axis.

14. Apparatus according to claim 1 together with a linear array of lightemitting diodes contained within the outline of said drum, and meansdirecting light from said diodes out of said drum, after reflection fromsaid reflecting surfaces, for imaging to comprise a reproduction of saidfield of view.

15. Apparatus according to claim 14 in which said detectors and saidoptical means are operative with infrared radiation, and said diodesemit visible radiation.

16. Means for scanning the field of view along a line of sightcomprising, in combination:

a hollow scanning drum rotatable about a first axis, including aplurality of inwardly reflecting plane surfaces substantially parallelto said axis;

means causing rotation of said drum;

first stationary optical means, including a first folding mirror, fixedwith respect to said drum and positioned to direct infrared radiationhaving the direction of said line of sight to impinge con- W QB PatentNo. 3,728,545 Dated April 17, 1973 Inventor(s) Irving 0 Abel It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Claim 1, line 7, after "first" (first occurence) insert --'a stationary;same line, after "first" (2nd occurence) cancel --station z a.ry;-; line15, after "second'f first occurence) insert stationary+-; I g 11m; after"se cbhii" (21nd occurehce) cahcei -sta- 7 Q line 16, cancel "tionary",

Signed and sealed this 27th day of November 1973';

(SEAL) Attest'z EDWARD M.FLETCHER,JR. RENE D. TEG'I MEYER" Attestlilg f1r Acti ng' Commissioner of- Patents FoRM Po-1o5o (10-69) IUSCOMM-DCaoalo-peo I a v.9, aovmnmn' "mime omcl an o-au-su

1. Means for scanning the field of view along a line of sightcomprising, in combination: a hollow scanning drum rotatable about afirst axis including a plurality of inwardly reflecting surfacessubstantially parallel to said axis; means causing rotation of saiddrum; first optical means including a first stationary folding mirror,fixed with respect to said drum and positioned to direct opticallyreflectable radiation having the direction of said line of sight toimpinge convergingly on said reflecting surfaces successively as saiddrum rotates; a linear array of radiation detectors fixed with respectto said drum; and second optical means, including a second stationaryfolding mirror, fixed with respect to said drum and arranged to receiveradiation reflected from said surfaces and direct it toward said array,whereby during rotation of said drum successive images of said field ofview sweep traversely across said array, to produce varying outputs fromsaid light detectors.
 2. Apparatus accordiNg to claim 1 in which saidsurfaces are plane.
 3. Apparatus according to claim 1 in which saidsurfaces are symmetrical about said axis.
 4. Apparatus according toclaim 1 in which said surfaces jointly define a regular polyhedralprism.
 5. Apparatus according to claim 1 in which said first opticalmeans also includes lens means for causing said radiation to impingeconvergingly on said surfaces.
 6. Apparatus according to claim 1 inwhich said first and second optical means are contained within theoutline of said drum.
 7. Apparatus according to claim 1 in which saiddetectors are outside the outline of said drum.
 8. Apparatus accordingto claim 1 in which said second optical means also includes a furtherreflector lying in the path of radiation between said first foldingmirror and said drum, said further reflector being of limited transversedimension to cause minimum obscuration of said light.
 9. Apparatusaccording to claim 6 in which said further reflector is a portion of thesurface of a cylinder of which the axis lies in a plane perpendicular tosaid first axis, and which is tangent to said first axis.
 10. Apparatusaccording to claim 1 including further means for causing said reflectedradiation to image on said array.
 11. Apparatus according to claim 5 inwhich said lens means includes a relay lens located within the outlineof said drum between said first folding mirror and said surfaces. 12.Apparatus according to claim 8 in which said further means includes astrip mirror located within the outline of said drum and a relay lens.13. Apparatus according to claim 1 in which said surfaces slightlydepart in opposite directions alternately from a parallel relation tosaid axis.
 14. Apparatus according to claim 1 together with a lineararray of light emitting diodes contained within the outline of saiddrum, and means directing light from said diodes out of said drum, afterreflection from said reflecting surfaces, for imaging to comprise areproduction of said field of view.
 15. Apparatus according to claim 14in which said detectors and said optical means are operative withinfrared radiation, and said diodes emit visible radiation.
 16. Meansfor scanning the field of view along a line of sight comprising, incombination: a hollow scanning drum rotatable about a first axis,including a plurality of inwardly reflecting plane surfacessubstantially parallel to said axis; means causing rotation of saiddrum; first stationary optical means, including a first folding mirror,fixed with respect to said drum and positioned to direct infraredradiation having the direction of said line of sight to impingeconvergingly on said reflecting surfaces successively as said drumrotates; a linear array of radiation detectors fixed with respect tosaid drum; and second stationary optical means, including a secondfolding mirror, fixed with respect to said drum and arranged to receiveradiation reflected from said surfaces and direct it toward said array,whereby during rotation of said drum successive images of said field ofview sweep traversely across said array, to produce varying outputs fromsaid light detectors.